Inorganic dual-layer microporous supported membranes

ABSTRACT

The present invention provides for a dual-layer inorganic microporous membrane capable of molecular sieving, and methods for production of the membranes. The inorganic microporous supported membrane includes a porous substrate which supports a first inorganic porous membrane having an average pore size of less than about 25 Å and a second inorganic porous membrane coating the first inorganic membrane having an average pore size of less than about 6 Å. The dual-layered membrane is produced by contacting the porous substrate with a surfactant-template polymeric sol, resulting in a surfactant sol coated membrane support. The surfactant sol coated membrane support is dried, producing a surfactant-templated polymer-coated substrate which is calcined to produce an intermediate layer surfactant-templated membrane. The intermediate layer surfactant-templated membrane is then contacted with a second polymeric sol producing a polymeric sol coated substrate which is dried producing an inorganic polymeric coated substrate. The inorganic polymeric coated substrate is then calcined producing an inorganic dual-layered microporous supported membrane in accordance with the present invention.

PRIORITY APPLICATION

[0001] This application claims priority to Provisional Application Ser.No. 60/141,148 filed Jun. 25, 1999.

ACKNOWLEDGEMENT OF GOVERNMENT RIGHTS

[0002] This invention was made with Government support at SandiaNational Laboratories under Contract No. DE-AC04-94AL85000 awarded bythe Department of Energy. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

[0003] The present invention relates to membranes for use in molecularsieving, and more particularly to inorganic membranes providing highsieving flux and selectivity.

BACKGROUND

[0004] Membrane-based separations are energy efficient and costeffective. They represent promising alternatives to energy-intensivedistillation, cryogenic separation, or pressure swing adsorption inapplications such as purification of sub-quality natural gas, airseparation, removal of VOCs and NO_(X), and hydrogen recovery fromprocessing gases and feed stocks. Microporous inorganic membranes haveattracted considerable attention for gas separation due to theirexcellent thermal and chemical stability, good erosion resistance andhigh pressure stability compared to conventional polymeric membranes(e.g. cellulusic derivative, polysulfone, polyamide, or polyimidemembrane).

[0005] An inorganic membrane system generally consists of a macroporoussupport providing mechanical strength for an overlying thin, eitherdense or porous, separation membrane. Dense membranes prepared frompalladium or perovskite only allow certain gases (such as H₂ or O₂) totransport via mechanisms such as solution-diffusion or solid-state ionicconduction. Such membranes require high capital investment due to theuse of precious metals and/or extreme synthesis conditions. In contrast,porous silica membranes with tunable pore sizes can be processed by asimple dip-coating or spin-coating procedure and can be used potentiallyin a large variety of gas separations. Microporous silica membranes havebeen demonstrated to show promising molecular sieving characteristics.

[0006] Certain techniques have been developed to process porous silicamembranes. They include sol-gel synthesis, leaching, and chemical vapordeposition. Among these, sol-gel processing attracts the most attentiondo to its excellent processibility and its potential to preciselycontrol pore size and pore structure. Strategies for the fundamentalphysical and chemical phenomena involved in the deposition of colloidalceramic dispersions (sols) on porous supports for precise pore size andporosity control have been proposed and discussed in Brinker et al.,“Sol-gel Strategies for Controlled Porosity Inorganic Materials,” J.Membr. Sci., 94 (1995) 85, incorporated herein by reference. Three keysto membrane production are 1) avoidance of cracks, pinholes or otherdefects that would reduce the selectivity; 2) precise pore size control(0.3- 0.4 nm in diameter) so that separation occurs on the basis of sizeby molecular filtration or “sieving”; and 3) maximization of the volumefraction porosity and minimization of the membrane thickness to maximizeflux.

[0007] Forming a microporous silica membrane on top of a home-madedisk-shaped, double-coated γ-alumina support has been described in thearticle De Vos et al., “Improved Performance of Silica Membranes for asSeparation,” J Membr. Sci., 143 (1-2) (1998) 37-51, incorporated hereinby reference. The coating and calcination process was repeated once tominimize potential defects. Both silica sol preparation and membraneprocessing were similar to those developed by De Lange, Hekkink andKeizer, described in De Lange et al., “Formation and Characterization ofSupported Microporous Ceramic Membranes Prepared by So-Gel ModificationTechniques,” J. Membr. Sci., 99 (1995) 57-75, incorporated herein byreference. The improvement of membrane performance was attributed to theprocessing under class-10 clean room conditions, which increases thecost of manufacturing. Prior art membranes have employedrepeated-coating processes to reduce intrinsic defects. The multi-stepcoating process results in reduction of the number of defect sites, thusincreasing selectivity but at the expense of permeation flux and cost ofmanufacturing. Although high selectivity can significantly reduce eitherfeed loss (single stage) or recompression costs (multiple stage), highpermeation flux is necessary to achieve commercially satisfactoryproduction rates. Therefore, the deadlock of tradeoff betweenselectivity and flux needs to be overcome.

SUMMARY OF THE INVENTION

[0008] The present invention provides for an inexpensive supportedmembrane capable of molecular sieving.

[0009] The present invention further provides for a uniform intermediatelayer on a substrate to allow the deposition of a second, topmicroporous layer which is relatively defect free and a method forproducing the defect free layer.

[0010] The present invention also provides for a supported membrane withprecise pore size to achieve molecular sieving with maximized flux andselectivity.

[0011] There is described a dual-layer inorganic microporous membranecapable of molecular sieving, and methods for production of suchmembranes. The inorganic microporous supported membrane includes aporous substrate which supports a first inorganic porous membrane havingan average pore size of less than about 25 Å and a second inorganicporous membrane coating the first inorganic membrane having an averagepore size of less than about 6 Å.

[0012] The dual-layered membrane is produced by contacting the porousmembrane support with a surfactant-containing inorganic polymeric sol,resulting in a surfactant/inorganic polymer coated membrane support. Thesurfactant/inorganic polymer coated membrane support is dried, producinga self-assembled surfactant-templated surfactant/inorganic polymercomposite film. This supported composite film is calcined to remove thesurfactant templates to produce a surfactant-templated micro- ormesoporous membrane substrate, which serves as an intermediate layersurfactant-templated membrane. The intermediate layersurfactant-templated membrane is then contacted with a second inorganicpolymeric sol producing an inorganic polymeric sol coated substratewhich is dried, producing a supported inorganic polymer coated duallayer structure. This supported dual-layered structure is then calcinedto produce a dual-layered microporous supported membrane in accordancewith the present invention.

[0013] In one embodiment, both of the polymeric sols include silica,aligomers or polymers. The first or intermediate layer of the dual-layersupported membrane generally has a thickness of less than 100 mn and thesecond or top layer has a thickness of less than about 100 Å. Theaverage pore diameter of the dual-layer supported membrane graduallydecreases in size from about 40 to 60 Å for the support, to about 10 to25 Å for the intermediate layer, to about 2 to 5 Å for the topmicroporous layer.

[0014] The calcining procedure of the second, top layer includescalcining under a vacuum of less than about 4 psia at a temperature ofbetween 200 to 400° C. and further calcining at a temperature of between300 to 600° C. The calcining of the first or intermediate layer involvescalcining at a temperature between 100 to 150° C. and further heatingbetween 500 to 600° C. The drying of the sols involves drying underconditions of low relative pressure of the liquid constituents.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and various other features and advantages of the presentinvention will be apparent upon reading of the following detaileddescription in conjunction with the accompanying drawings, where:

[0016]FIG. 1 is a graphical representation of weight loss anddifferential thermal analysis showing an endotherm peak corresponding toweight loss, indicating decomposition of the surfactant template and theoxidative pyrolysis of surfactant and residual organics.

[0017]FIG. 2 is a graphical representation of nitrogen adsorptionisotherms of three different calcined sol layers after removal oftemplates.

[0018]FIG. 3A is an Scanning Electron Microscope (SEM) cross-sectionalelectron micrograph of the dual-layer supported membrane of the presentinvention.

[0019]FIG. 3B is a zoomed in Transmission Electron Microscope (TEM)micrograph of the cross-sectional view of the dual-layer supportedmembrane of FIG. 3A.

[0020]FIG. 4 shows a schematic diagram of the custom built automatedflow system utilized in gas permeation measurements.

[0021]FIG. 5A is a graphical representation of the separation factorversus temperature comparing microporous sieving A2** layer with andwithout a surfactant-templated intermediate layer.

[0022]FIG. 5B is a graphical representation of gas permeances versustemperature comparing the A2** membrane with or without a mesoporoussublayer.

[0023]FIG. 6 is a graphical representation of the CO₂ permeance andCO₂/CH₄ separation factor α as a function of temperature for separationof 50/50 (v/v) CO₂/CH₄ gas mixture.

[0024]FIG. 7A is a graphical representation of the gas permeances versustemperature of the BTE/TEOS membranes with or without asurfactant-templated intermediate layer.

[0025]FIG. 7B is a graphical representation of the selectivities versustemperature of the BTE/TEOS membranes with or without asurfactant-templated mesoporous C16STS intermediate layer.

[0026]FIG. 8 is a graphical representation of the gas permeance versusmolecular diameter of a variety of molecules of a dual-layer supportedmembrane after calcination at 450° C.

[0027]FIG. 9 is a graphical representation of the separation factorversus permeance of the dual-layer supported membrane of the presentinvention in comparison to prior art membranes.

[0028]FIG. 10A is a graphical representation of the %CO₂ in permeate andCO₂/CH₄ separation factor versus stage cut of a dual-layer supportedA2** membrane.

[0029]FIG. 10B is a graphical representation of the CO₂ permeance andCO₂/CH₄ separation factor versus feed flow-rate of a dual-layersupported A2** membrane.

[0030]FIG. 11 is a graphical representation of the CO₂ permeance andCO₂/CH₄ separation factor versus change in pressure of a dual-layersupported A2** membrane.

[0031]FIG. 12 is a graphical representation of the CO₂ permeance andCO₂/CH₄ separation factor versus time of operation of a dual-layersupported A2** membrane.

DETAILED DESCRIPTION

[0032] In one embodiment, the present invention provides for novelinorganic dual-layered microporous supported membranes and methods forfabricating the supported dual-layer membranes. The dual-layer membranesare constructed by utilizing an underlying support structure coated witha surfactant-templated micro or mesoporous intermediate layer, thencoated with a top selective microporous layer. In the production ofsupported membranes, the quality of the underlying support determines,to a high degree, the properties and quality of the top selectivemicroporous silica membrane. Supports with rough and large-pore surfacescan cause overlying derived membranes to crack due to stress developmenton uneven film coatings or serve to create pin-holes (areas not coatedby the overlying microporous layer). Therefore, the present inventionprovides a novel and efficient way for the modification of supportsurfaces to allow a stable, even, and substantially defect-freesubsequently deposited overlying microporous membrane. Mechanicalpolishing to remove surface roughness is tedious, requiring repeatedprocesses, and is difficult to perform within the interior of tubularsupports. In the present invention, a membrane, such as a tube-sideγ-alumina surface of a commercial asymmetric membrane support, ismodified with a layer of surfactant-templated material, for example,surfactant-templated microporous or mesoporous silica (STS). The STS,configured with high porosity and narrow pore diameter distribution(˜1-2 nm), is designed to minimize additional flow resistance, surfaceroughness, and inherent support defects. It also reduces penetration ofa subsequently deposited top layer into the support, thereby reducingthe effective thickness of the top layer. This new dual-layer approachcreates a thin defect-free microporous membrane with both higherselectivity and higher flux than achieved by the correspondingsingle-layer microporous membrane.

[0033] The present invention provides for supported inorganic membranescapable of molecular sieving, as well as methods for their preparationand use. In one embodiment, the subject inorganic membranes have poreswith resulting average pore diameters of less than about 10 Å andusually less than about 5 Å and have a narrow pore diameterdistribution. The subject membranes are prepared by initially coating aporous substrate with a surfactant-templated inorganic polymeric sol.After deposition, the surfactant sol coated substrate is dried andcalcined to provide the intermediate inorganic membrane. Theintermediate inorganic membrane is then coated by a second polymericsol. After deposition, the inorganic polymeric sol coated substrate isdried, preferably under conditions of low relative pressure of theliquid constituents of the sol, and calcined to provide the subjectinorganic dual-layered microporous supported membrane.

[0034] The inorganic polymeric sols which find use in the presentinvention are those sols which are stable under the conditions in whichthey are employed, and have a viscosity which provides for deposition ofa thin sol coating on the support. The viscosity of the sols willgenerally range from about 1 to 20 cps, usually from about 1 to 5 cps,more usually from about 1.2 to 1.5 cps.

[0035] The polymeric sols will comprise a colloidal dispersion ofinorganic polymers in a liquid, where the liquid may be a singlecomponent, e.g. H₂O or an alcohol, or a multi-component, normallymiscible, mixture, e.g. alcohol-water and the like. The inorganicpolymers will often have a low mass fractal dimension (D_(f)) and besufficiently large so that the inorganic polymers are captured, at leastin part, on top of the underlying STS substrate. The diameter of theinorganic polymers as measured by small angle scattering will often beat least equal to, and will usually be greater than, the diameter of thepores in the underlying STS substrate, and may be up to 2 times thediameter, more usually up to 1.5 times the diameter of the pores of theunderlying STS substrate. The concentration of the inorganic polymers inthe liquid component of the sols will be sufficient to provide for a solwith the desired viscosity, as described above. Generally, theconcentration of the inorganic polymers in the liquid component of thesols will range from 0.45 to 0.69, usually 0.59 to 0.63, more usually0.60 to 0.62 moles/liter.

[0036] The liquid component of the sols will often comprise water incombination with at least one, usually not more than three, more usuallynot more than two, organic solvents. Water present in the liquidcomponent will usually be deionized and will be present in about 2.5% to13% (v/v), usually 5.7 to 6.0% (v/v) of the total liquid component ofthe sols. A variety of organic solvents will normally be combined withthe water to produce the liquid component of the sols. At least one ofthe organic solvents will be an alkanol, usually a lower alkanol of from2 to 8 carbon atoms, usually 2 to 6 carbon atoms, more usually 2 to 4carbon atoms. The alkanol may be an alcohol or polyol, where the numberof hydroxy groups does not exceed the number of carbon atoms, and wherethere is usually not more than one hydroxy group for every 1.5 carbonatoms. Illustrative alkanols which find use as organic solvents in thesubject sols are ethanol, usually absolute ethanol, methanol,iso-propanol, and the like. The alkanol will be present in the liquidcomponent of the sols in an amount ranging from about 77 to 82% (v/v),usually from about 80 to 80.5%(v/v) of the liquid component in the sols.Generally, the ratio of alkanol to water in the liquid component of thesols will range from 6 to 32, usually from about 13.5 to 14.5. Otherorganic solvents, in addition to the alkanol, may also be present in thesubject sols, where the liquid component of the sols comprises more thanone organic solvent. These other organic solvents may be straightchained, branched or cyclic, and will usually comprise from about 2 to10 carbon atoms, more usually 4 to 8 carbon atoms. The organic solventsmay comprise one or more heteroatoms, usually not more than threeheteroatoms, where the heteroatom may be selected from oxygen, sulfur,nitrogen and the like. Illustrative non-alkanol organic solvents whichmay be present in the liquid component of the sols include hexane,toluene, tetrahydrofuran, acetone and the like. When present in thesubject sols, these non-alkanol organic solvents will be present inamounts ranging from 5 to 99% (v/v) of the total liquid component.

[0037] The subject inorganic polymeric coating sols are prepared bycombining metal alkoxide monomers with the liquid components of thesols, as described above, in the presence of a catalyst, either acid orbase, with agitation to provide a substantially uniform dispersion ofthe metal alkoxide monomers in the liquid components. Metal alkoxidemonomers that find use in preparation of the subject sols are thosemonomers having the formula:

M(OR)_(n) or MR′_(x)(OR)_(n−x), usually M(OR)_(n),

[0038] wherein:

[0039] M is a metal having a coordination number in excess of 3, i.e.>3,and is selected from the group comprising Si, Al, Ti, Zr and the like,preferably Si;

[0040] OR is a hydrolyzable alkoxy ligand, where R usually has theformula:

C_(z)H_(2z+1),

[0041] wherein:

[0042] z is from 1 to 4, usually from 1 to 2;

[0043] n is usually from 3 to 4; and

[0044] R′ is a non-hydrolyzable organic ligand or other oligomericoxoalkoxide having from 1 to 4 carbon atoms, usually from 1 to 2 carbonatoms.

[0045] For electropositive metals, e.g. Ti and Zr, the metal alkoxidemonomer may be modified to reduce the effective functionality and/orrate of hydrolysis and condensation and thereby prevent particleformation under the sol preparation conditions. To reduce the effectivefunctionality and/or rate of hydrolysis and condensation, the metalalkoxides may be modified by any convenient means, such as chelationwith slowly hydrolyzing multidentate ligands, e.g. actylacetonate,alcohol amines, and the like.

[0046] Catalysts that find use include mineral acids and bases,including HCl, H₂SO₄, HNO₃, NH₄OH, and the like. In the presentinvention, often the catalyst is added in sufficient concentration tocause the average condensation rate of the hydrolyzed alkoxide speciesto be minimized.

[0047] Agitation of the metal alkoxide, water, catalyst and organicsolvent(s), i.e. the sol precursors, is maintained for a sufficient timeto provide for a uniform dispersion of the metal alkoxide in the liquidconstituents. Usually the sol precursors will be mixed for a period oftime ranging from 5 to 20 min, more usually 14 to 15 min. Mixing may beaccomplished by any convenient means, such as stirring, shaking and thelike.

[0048] The resultant mixture of sol precursors is then allowed to set orage for a sufficient time under conditions of low condensation rate toproduce a sol comprising inorganic polymers having the desired clustersize. The condensation conditions, i.e. the rate at which the monomersare hydrolyzed and then condensed into the polymeric clusters, dependson a variety of factors, such as the reactivity of the alkoxy ligands ofthe metal alkoxide monomers, the temperature at which the sol ismaintained, the pH of the sol precursor, and the like. Thus, thetemperature and pH of the combined sol precursors will be selected inview of the reactivity of the alkoxy ligands of the metal alkoxymonomers, to achieve conditions of low but finite condensation rate.Although the selected temperature and pH will vary depending on theparticular metal alkoxy monomers employed, generally the temperaturewill range from about 35 to 70° C., usually from about 40 to 60° C., andmore usually from about 48 to 52° C. The temperature of the agingprecursor sol may be controlled using any convenient temperature controlmeans, such as a heating or cooling means, or the like. The effective pHof the sol precursor composition will generally range from 0 to 5,usually from about 1 to 3, more usually from about 1.5 to 2.5. Forsilica sols, the pH of the precursor solution may be controlled byintroducing to the precursor an acid or basic acatalyst, such as HCl,HNO₃, NH₄OH and the like, in sufficient amount to modulate the pH(specifically the −log[H₃O⁺]) of the precursor to the desired value.

[0049] The precursor sol is sealed and allowed to set or age under theconditions of low condensation rate for a period of time sufficient toprovide for the formation of extended polymeric networks capable ofinterpenetration and optionally having the desired fractal dimensions.Usually, the sols will be allowed to set in a closed container for aperiod of time ranging from about 0 to 140 hrs, more usually from about10 to 24 hours.

[0050] The preparation of a silica polymeric coating sols includes a twostage process, in which the pH of the sols is decreased from the firstto the second stage. In the first stage of this two stage sol productionprocess, a silicon alkoxide, e.g. tetraethoxysilane, is combined with analkanol and water with agitation to produce a precursor sol. The amountof silicon alkoxide which is combined with the alkanol and water toproduce the precursor sol will range from 1.0 to 2.0 mol/liter, usuallyfrom about 1.7 to 2.0 M mol/liter. The amount of alkanol present in theprecursor sol will range from 30 to 70% (v/v), usually 45 to 50% (v/v),and the amount of water present in the precursor sol will range from 2to 10% (v/v), usually 3.5 to 4.5% (v/v). In this first stage, agitationis continued for a period of 30 to 120 min., usually 60 to 90 min.During this first stage, the pH of the precursor sol is maintained at avalue between 3 and 6, usually between 4 and 5 as measured by using anindicator strip, by including in the precursor sol a sufficient amountof an acid catalyst. Any convenient acid catalyst may be employed,including HNO₃, HCl and the like. In this first stage, the molar ratioof the four components of the sol precursor, i.e. silicon alkoxide:alkanol: water: acid catalyst, will be 0.8-1.2 : 3.5-4 : 0.8-1.2:1.0×10⁻⁵-9.0×10⁻⁵.

[0051] In the second stage of the two stage sol preparation process, thepH of the precursor sol will be reduced by ⅓to ½. To reduce the pH ofthe precursor sol, a sufficient amount of water and acid catalyst willbe introduced to the precursor sol. Following introduction of the waterand acid catalyst, the pH of the precursor sol will be reduced tobetween about 1 and 3, usually between about 1.5 and 2.5. In the secondstage, a sufficient amount of water and acid precursor are added to theprecursor sol to change the molar ratio of the four components to0.8-1.2: 3.5-4: 4.5-5.5: 0.001-0.009. Following introduction of the acidcatalyst and water, the precursor sol will be agitated for a period oftime ranging from 10 to 30 min, usually from 10 to 20 min. Followingagitation, the precursor sol will be allowed to set or age at anelevated temperature for a period of time sufficient for the coating solto be produced. Generally, the incubation temperature will range from 40to 60° C., usually from 45 to 55° C. The setting period will range from12 to 36 hours, usually 12 to 24 hours. The resultant silica coating sol(referred to below as the A2** sol) may comprise silica polymers of lowfractal dimension.

[0052] In one embodiment of the present invention, thesurfactant-template polymeric sol utilized to form the intermediatelayer membrane is further prepared from the precursor sol followingsubstantially similar steps as described above for creating the A2**sol. However, prior to aging and the second stage, a sufficient amountof surfactant powder is added to the unaged precursor sol. Surfactantpowders that are used are usually cationic surfactants, for exampleC6-surfactant of triethylhexylammonium bromide and C16-surfactant ofcetyltrimethylammonium bromide. The amount of surfactant powder that isadded is between 2 g and 8 g per 100 g sol, usually between 3 g and 5 gper 100 g/sol. Following the addition of the surfactant powder,precursor sol is agitated for a period of time between about 5-30 min.,usually between about 10-20 min. Following the addition of thesurfactant powder, the surfactant-template sol is completed followingsubstantially similar steps as those of aging and the second stage,described above for the A2** sol.

[0053] In one embodiment, the polymeric sols utilized for both theintermediate layer and the top or outer layer are formed from the sol ora derivative of the sol labeled A2** and fully described in detail inU.S. Pat. No. 5,772,735, incorporated herein by reference.

[0054] In alternative embodiments, a sol providing substantially similarsieving characteristics as the A2** sol is utilized as the topmicroporous membrane. The alternative sol consists of five components,i.e., Bis(Triethoxysilyl)Ethane (BTE):siliconalkoxide:alkanol:water:acid catalyst, with a molar ratio being0.4-1.2:0.1-0.5:3.5-4:4.8-5.4:1.0×10³-9.0×10^(3.) Further, alternativesto the STS sol include sols containing amphiphilic block-copolymers. Theamphiphilic block-copolymers, for exampleBrij56,CH₃(CH₂)₁₅(CH₂CH₂O)₁₀OH, 4 wt %, are utilized as the templatingagent in replace of the surfactant in creating the intermediate sollayer, producing ordered mesoporous silica sublayer membranes. Theblock-copolymer templated silica provides both a high porosity ofbetween 40 to 80%, usually between 50 to 60%, and an ordered cubicstructure, resulting in a membrane having an average pore diameter ofless than about 30 Å, usually less than 25 Å.

[0055] In one embodiment, foreign particles are removed from the sols(both the intermediate and top layer sols) through the filtering of thesols prior to being coated onto a substrate. The top layer or A2** solsare filtered with a 0.2 to 1.0 μm filter, more usually with a 0.45 82 mfilter. The surfactant-template sols are filled with a 0.45 to 2.0 μmfilter, usually with a 1.0 μm filter. Thus, potentially damaging foreignmaterials are removed from the sols to further ensure membranes withaccurate sieving capabilities.

[0056] A variety of substrates may be employed as porous supports forthe inorganic membranes. Substrates that may be employed have pore sizeswhich are sufficiently large such that the substrate itself does notcontribute to the sieving properties of the supported membrane.Generally, the pores of the substrate will have diameters at least 5times larger than the pores of the inorganic films to be deposited onthe substrate, normally at least 8 times larger, and not more than 15times larger, usually not more than 12 times larger than the pores ofthe inorganic film to be deposited. The substrate pores usually havediameters ranging in size from about 30 to 70 Å, usually from about 40to 60 Å.

[0057] The substrate is constructed of any suitable material that isthermally, chemically and mechanically stable during sol deposition,thermal processing and membrane use including oxides, e.g. TiO₂, Al₂O₃,ZrO₂, hydroxides, e.g. AlOOH, as well as porous metals, such asstainless, and other materials know in the art. The substrates may haveany convenient shape, such as square, rectangular and cylindrical, aswell as other more complicated shapes, where the shape chosen willdepend primarily on the intended use of the final supported inorganicmembrane. The substrates may be prepared using methods known in the art,i.e. deposition of particulate sols, or obtained from commercialsources. Commercially available substrates capable of acting as supportsfor the inorganic films of the subject invention include those availablefrom U.S. Filter, e.g. Membralox® (a cylindrical substrate comprising aγ-Al₂O₃ inside layer having pores ranging in diameter from 40 to 50 Å),Golden Technologies, and the like. As necessary and desirable, prior touse, the substrate may be calcined to eliminate any organic compoundspresent on the support and to desorb any water from the support.Calcination will generally be carried out at temperatures ranging from300 to 500° C., usually 350 to 450° C., for a period of time between 15to 180 min., and usually between 45 to 75 min.

[0058] Prior to depositing the STS sublayer onto the membrane supports,the membrane supports are cleaned, usually by ultrasound. The membranesupports are then washed with de-ionized water 1 to 7 times, usually 3to 5 times. The membrane supports are then calcined at a temperaturebetween 300 and 500° C., usually 350 to 450° C., for a period of timebetween 15 to 180 min., and usually between 45 to 75 min.

[0059] The STS sol is then deposited onto the surface of the membranesupport under substantially clean conditions, usually under Class 100clean conditions, to produce a surfactant sol coated support that isthen dried and calcined to yield an STS inorganic intermediate membrane.The drying is performed in an environment having between 10-30% relativehumidity, usually having 15-25% relative humidity at a temperaturebetween 10 and 40° C., usually between 20 and 30° C. for a period oftime between 5 and 20 minutes, usually between 10 and 15 minutesresulting in a surfactant-template polymeric coated substrate. Thecalcination is performed by heating the surfactant-template polymericcoated support at a temperature between 50-200° C., usually between100-150° C. for between 30-120 min., usually between 45-75 min.producing a surfactant-template membrane support. Surfactant removal isthen performed by heating the STS membrane support at a temperaturebetween 300 to 700° C., usually between 500-600° C. for between 30-90min., usually between 45-75 min. producing the intermediate layersurfactant membrane. Deposition of the surfactant intermediate orsublayer is achieved by contacting the membrane support with an STSsol-gel so that the STS sol coats the membrane support in a layercapable of collapsing with drying to produce a thin film. The supportmembrane may be contacted with the STS sol using any convenient means,such as dip-coating, infiltration, spin-coating, spraying and the like.

[0060] In one embodiment, the A2** silica top layer is then depositedonto the surface of the intermediate layer surfactant membrane, using asimilar procedure as described above, producing an A2** polymeric solcoated substrate that is then dried and calcined. The drying isperformed in an environment having between 10-30% relative humidity,usually having 15-25% relative humidity at a temperature between 10 and40° C., usually between 20 and 30° C. for a period of time between 5 and20 minutes, usually between 10 and 15 minutes producing an A2**inorganic polymer coated substrate. The calcination of the A2** coatedsubstrate is performed under a vacuum of less than about 6 psia (poundsper square inch absolute), usually less than about 4 psia (<4 psia), ata temperature between 200 and 400° C., usually between 250 and 350° C.During calcination, the rate at which the temperature is raised willrange from 0.5 to 5° C./min., usually 0.5 to 2° C./min. The coatedsubstrate will be held isothermally at the calcination temperature for aperiod of time ranging from about 4-8 hours, usually 5-7 hours afterwhich the temperature will be decreased at a rate of 0.5 to 5.0°C./min., usually between about 0.5 to 3.0° C./min. to produce adual-layered inorganic supported membrane having the A2** topmicroporous membrane. In an alternative embodiment, further calcinationor second calcination is performed on the A2** supported membrane at atemperature between 300-600° C., usually between 400-500° C. in air for30 to 90 min., usually 45 to 75 min. producing the final A2** inorganicdual-layered microporous supported membrane. The ends or edges of thefinal permeable membrane are then sealed to prevent any defects orirregularities.

[0061] The final porous inorganic membranes are capable of molecularsieving, and exhibit high flux and high selectivity. By capable ofmolecular sieving is meant that the membranes are capable of exhibitingsubstantially molecular sieving behavior, where molecular sievingbehavior exists when the separation factor for a pair of gasestransported through the membrane is greater than the separation factorfor the same pair of gases in a membrane characterized by Knudsendiffusion. Thus, in the formula where α_(A/B)∝(molecular weightB/molecular weight A)^(½), where α_(A/B) is the separation factor of amembrane for a pair of gases, in membranes exhibiting Knudsen separationbehavior, the separation factor for the pair of gases does not typicallyexceed 10. As the subject membranes are capable of exhibitingsubstantially molecular sieving behavior, the separation factor forpairs of gases will in general greatly exceed the Knudsen separationvalues.

[0062] The supported inorganic membranes are substantially defect free,in that they are substantially free of cracks, pinholes and the like. Bysubstantially defect free is meant that the supported inorganicmembranes are at least 95% defect free, usually at least 97% defectfree, more usually at least 99% defect free. The supported inorganicmembranes are thin, ranging in thickness from 10 to 200 nm, usually from15 to 150 nm and more usually from 20 to 100 nm. Preferably, thethickness of the supported inorganic membranes will be less than about100 nm. The supported inorganic membranes of the subject invention willhave a narrow pore diameter distribution, i.e. there will be littlevariance in pore diameter. The pore diameter of the subject membraneswill be sufficiently small to provide for size exclusion of molecules,i.e. sufficiently small to provide for selective passage of smallermolecules while blocking larger molecules. The STS intermediate layermembrane will have pores with an average diameter, dependent upon thesurfactant utilized, generally in the range of about 5 to 30 Å, moreusually from about 10 to 25 Å. The A2** membrane will have pores with anaverage diameters generally in the range of about 1 to 1 Å, usually fromabout 2 to 5 Å.

[0063] Some of the critical issues in the processing of the sol-gelderived silica membranes include eliminating defect formation andcontrolling pore size. Described below are some of the strategiesemployed in one embodiment of the membrane formation to eliminate defectformation. First, drying-induced stress as high as 200 MPa in the silicasol system can result in film cracking unless the film thickness isbelow a critical cracking thickness h_(c). Through the adjustment of thesol concentration, withdrawal rate or sol aging time to maintain themembrane film thickness below the critical thickness h_(c) (˜4000 Å forA2** sol), cracking is avoided. Second, the membrane is processed underclean conditions to substantially avoid foreign particles causingpinholes. Third, STS intermediate layer is used to eliminate inherentdefects on commercially-available supports and to facilitate theformation of subsequently deposited thin selective membranes (e.g. A2**membrane). The STS intermediate layer is designed with both highporosity and low tortuosity to avoid creating additional flowresistance. Examples of strategies for pore size control, includesolvent (water) templating, and surfactant-templating. Due topreferential alcohol evaporation during the film deposition or solcontacting process, water is the dominating solvent at the late dryingstage. Water molecules confined in the stressed film are used as atemplate to create pores of molecular dimension needed for molecularsieving. For the surfactant-templating strategy, surfactants, agregatesor liquid crystalline mesophases (amphiphilic molecules composed of ahydrophilic head group and hydrophobic tail) are used as templates.Surfactant-templated silicas are high surface area amorphous solids (upto approximately 1400 m²/g) characterized by monosized, oftencylindrical pores organized into periodic arrays that often mimic theliquid crystalline mesophases exhibited by surfactant-water systems.Both water and surfactants can be removed by heating to create uniformpores with various dimensions depending on a choice of surfactant.

[0064] The resulting inorganic dual-layer membranes find use in avariety of applications, including purification of sub-quality naturalgas, removal of NO_(x) from power-plant flue gas, reduction ofgreenhouse gases (e.g. CO₂) and hydrogen recovery from processing gasesor hydrogen purification for fuel-cell applications. The highly :selective top layer will, for example, remove N₂ and CO₂ efficientlyfrom natural gas without suffering from CO₂ plasticization commonly seenin dense polymeric membranes or purify H₂ from reformate for fuel-cellapplications.

[0065] The following examples are offered by way of illustration and notby way of limitation.

EXPERIMENTAL EXAMPLE 1

[0066] Synthesis of Micro-/Mesoporous Silica Materials

[0067] 1.A. Preparation of Silica Polymeric Sols

[0068] 1.A.1.—A silica polymeric sol (labeled A2** ) was prepared asfollows. Preparation of the A2** sol consists of two acid-catalyzedreaction steps designed to minimize the condensation rates of silicaspecies in order to produce weakly branched polymeric clusters thatinterpenetrate and collapse during film deposition to produce membraneswith molecular-sized pores. In the first step an A2** stock solution wasprepared from tetraethoxysilane (TEOS) (Kodak), 200 proof ethanol(EtOH), deionized H₂O, and 0.07 N HCl (diluted from ‘Baker Analyzed® 1 NHCl). The four components were mixed in a molar ratio of1.0:3.8:1.1:5.0×10⁻⁵, respectively, and refluxed at 60° C. for 90 min.while stirring at 200 rpm (PMC 730 series DATAPLATE® or a WhatmanDATAPLATE® model 440 P programmable digital hot plate/Stirrer, 500 mlPyrex® resin reaction kettle with lid, Pyrex® 24/40 condenser). The pHof the prepared stock solution was measured using EM Science ColorpHast®pH paper and found to be 4.7. This A2** stock solution is very stablefor approximately 90 to 180 days when stored at −30° C. In the secondstep, additional water and HCl were added into the stock sol. The solwas agitated or hand-shaken for 15 minutes.

[0069] 1.A.2.—The sol is then aged at 50° C. for 22 hours. The two-stepprocedure resulted in an A2** standard sol with a final molar ratio ofTEOS: EtOH: H₂O: HCl=1.0:3.8:5.0:0.004(pH=2.0).

[0070] 1.A.3.—A dip-coating sol was prepared by diluting the A2**standard sol with two times its volume of ethanol. The dip-coating A2**sol was filtered with 0.45 μm filter (Nalgene) prior to dip-coating.

[0071] 1.A.4.—An alternative silica species to the A2** sol which willalso produce membranes with molecular-sized pores is a BTE/TEOS sol. Thedip-coating BTE sol is prepared as described above in 1.A.1. with thesol comprising BTE(Bis(Triethoxysilyl)Ethane):TEOS:ethanol:H₂O:HCl in amolar ratio of 0.8:0.2:3.8:5.1:5.3×10⁻³.

[0072] 1.B.—Synthesis of Surfactant-Templated Micro-/Mesoporous SilicaMaterials

[0073] 1.B.1.—Synthesis of a C6STS Microporous Silica Material. Asurfactant-template silica (STS) sol containing 0.125M C6-surfactant(triethylhexylammonium bromide, Aldrich) silica sol is prepared byadding 0.4 g per 100 g/sol amount of surfactant powder into unaged A2**standard sol as prepared in 1.A.1. The C6STS sol is then aged accordingto the process of 1.A.2. The C6STS sol is then diluted as described in1.A.3. with two times its volume of ethanol. The aggregation of thesurfactant during drying does not cause any macroscopic phaseseparations. The C6STS sol is filtered with a 1.0 μm filter (Nalgene)prior to dip-coating.

[0074] 1.B.2.—Synthesis of a C16STS Silica Material. Asurfactant-template silica sol containing a 4.2 wt % C16-surfactant(cetyltrimethylammonium bromide) is prepared by adding 0.4 g per 100g/sol amount of surfactant powder into unaged A2** standard sol asprepared in 1.A.2. and follows the process described in preparing theC6STS sol in 1.B.1.

[0075] 1 .B.3.—Characterization of the STS Intermediate Layer. Referringto FIG. 1, for the C6STS xerogel as prepared in 1.B.1., differentialthermal analysis (DTA) showed an endothermic peak near 200° C.corresponding to the beginning of a drastic weight loss indicating thedecomposition of the C6-surfactant template while an exothermic peak ataround 350° C. accompanied by a weight loss of about 45%, signified theoxidative pyrolysis of surfactant and residual organics. Referring toFIG. 2, N₂ sorption isotherms of the calcined C6STS thin film (calcinedat 500° C. for one hour), characterized by a surface-acoustic wave (SAW)technique, appeared to be Type I, characteristic of microporousmaterials; while that of calcined C16STS thin film as prepared in 1.B.2.appeared to be Type II, characteristic of mesoporous materials. Stillreferring to FIG. 2, although N₂ sorption isotherms of bulk calcinedA2** xerogel powder appeared to be Type I with a very sharp increase ofN₂ volume adsorbed within a relative pressure of 0.0 to 0.01, indicatinga narrow pore size and pore size distribution, a thin-film form wasvirtually inaccessible to N₂ at 77K. due to differences in drying rates(films dry much more rapidly). Average pore diameters of the C6STS andC16STS were about 10-12 Å and 18-20 Å, respectively. Furthermore, thesurface area and porosity of the C6STS materials were 575 m²/g and 28%,respectively. Porosity, determined by ellipsometry, agreed with theresult calculated from N₂ adsorption isotherms.

[0076] 1.B.4.—Synthesis of a Block Copolymer templated Silica Material.A Brij56, CH₃(CH₂)₁₅(CH₂CH₂O)₁₀OH, 4 wt %, amphiphilic block copolymersilica sol is prepared by adding 4 g/100 g sol amount of amphiphilicblock copolymer to the unaged A2** standard sol as prepared in 1.A.2.and follows the process described in preparing the C6STS sol in 1.B.1.

[0077] 1.C.—Sol Deposition

[0078] Membrane supports were prepared by sectioning a commercial 50 Åγ-Al₂O₃ tube (US filter) into several 5.5 cm-long sections and cleanedby ultrasound. The supports were then washed with de-ionized water 5times before calcining at 400° C. for 60 min.

[0079] 1.C.1—Surfactant Sublayer Coating of the Support. A sol-geldip-coating process featuring aspects of slip casting was performed in alaminar flow (150 ft/min) chamber under clean conditions of Class 100.To prepare a C6STS sublayer, the support tube as prepared and washed in1.C. was lowered into the sol containing the C6-surfactant as preparedand described above in 1.B.1. at a rate of 8 inches/min. The support washeld undisturbed for ten seconds, the support was then withdrawn at therate of 3 inches/min. The surfactant coated support was then dried at atemperature of 25° C. in a 20% relative humidity environment for 15minutes resulting in a surfactant-template polymeric layered membranesupport. The membrane was then calcined to 120° C. in a programmablefurnace. In performing the calcination, the membrane support was heatedat a rate of 1° C./min to the target temperature 120° C. and was heldisothermally for 60 min. The coated membrane support was then cooled ata rate of 1° C./min. to 25° C. The cooled membrane support was thenconfirmed to be substantially impermeable to helium by performing thepermeance measurements. The coated support was then subjected tosurfactant removal by heating the membrane tube in the programmablefurnace at a rate of 1° C./min. to a temperature of 500° C. resulting inthe intermediate surfactant membrane support. The membrane was heldisothermally for 60 min. then cooled at a rate of 1° C./min. to 25° C.The same procedure could also be used to prepare membranes with othersurfactant-templated silica sublayers.

[0080] 1.C.2—A2** Layer Coating of the Membrane Support. The supporttube with the C6STS layer as prepared in 1.C.1. was lowered into theA2** sol as prepared in 1A. at a rate of 10 inches/min. and leftundisturbed for 10 seconds. The membrane support was then withdrawn at arate of 1 inch/min. The A2** polymeric sol coated substrate was thendried at a temperature of 25° C. in a 20% relative humidity environmentfor 15 minutes resulting in an A2** layered membrane support. The A2**layered membrane support was then calcined in a programmable furnaceunder a vacuum condition of less than 4 psia (<4 psia) at a heating rateof 1-2° C./min from room temperature to 300° C. The A2** coated membranewas held isothermally for six hours at 300° C. to evacuate solvents andalso promote further pore shrinkage. The A2** coated membrane is thencooled at a rate of 1° C./min. to 25° C. The vacuum calcinationprocedure also results in the decomposition of surface ethoxy groups;therefore, a hydrophobic inner pore surface is formed as evidenced by anincrease in the water contact angle of membrane surface from 17° to 41°C. The imperfections near both ends of the final A2** dual-layered tubemembrane caused by being clamped in the laminar flow chamber were thensealed by dense silicone (Duraseal 1529, Cotronics Corp., N.Y.). Thefinal permeable length of the A2** coated tube is 4 cm.

[0081] 1 .C.3.—Further calcined A2** membrane. The dual layered membraneas prepared above in 1.C.2. was further calcined, prior to the coatingof the imperfections near both ends, at 450° C. by heating the membraneat a rate of 1° C./min. to the target temperature of 450° C., then heldisothermally for 60 min. in air and cooled at a rate of 1° C./min. to25° C.

[0082] The dual silica coated permeable tube results in both high fluxand selectivity for the membranes with gradual changes of pore size from50 Å (commercial γ-alumina support layer) to 10-12 Å(surfactant-templated silica sublayer), and then to 3-4 Å(ultramicroporous silica top-layer). Surfactant templates embedded inthe silica framework are removed by calcination while solvent templatesare evacuated by a low-temperature thermal treatment under vacuum. FIGS.3A and 3B show an electron micrograph view of the cross-section and thesurface of the supported dual-membrane. The thickness of the defect-freeultramicroporous A2** layer is about 30 nm.

EXAMPLE 2

[0083] Sublayer Effect On Permeation And Separation

[0084] 2.A.—Test Parameters

[0085] The gas permeation through the dual-layer membrane produced in1.C. was measured using a custom built automated flow system as shown inFIG. 4. An outgassing procedure is performed prior to measuring thepermeation because ambient moisture can easily condense inside silicamicropores. The outgassing procedure was conducted at 80° C. for threehours with dry helium purging across the membrane. The procedure fordetermining gas permeance included evacuating both sides of the membraneand then introducing pure gas or mixed gas into the tube side of themembrane. Gases were chosen to give a range of gas molecule sizes.Single components of He, H₂, CO₂, CO₂, N₂, and CH₄, and either its dualor multiple component mixture were tested. The pressure at the tube sidewas maintained at a constant pressure (6.5 bar). Meanwhile, theshell-side pressure was gradually built up due to permeated gases. Uponexceeding atmosphere pressure, the shell side was exposed to ambientpressure. To simulate practical operation, no sweeping gas was used inall experiments and, therefore, the problems of back diffusional fluxwere eliminated.

[0086] The flow rates of all inlet and outlet streams were directlymeasured by digital bubble flow meters (Humonics). If the permeate flowrate was below the detectable limit of the digital bubble flow meter (˜1cc/min), it could be calculated from the rate of pressure increase at apre-vacuumed permeate side with a closed outlet.

[0087] For mixed-gas permeation measurement, a pre-mixed gas with knowncomposition was used and the compositions of both effluent streams wereanalyzed as functions of time using an on-line gas chromatograph. Theflow rate and the pressure of the tube-side stream as well as thetemperature of the membrane separator were varied. The experiments werecontinued until steady-state conditions (no change in flow rates andcompositions with time) were reached. The change of gas-phase drivingforce (the partial-pressure gradient of component i, ΔP_(i), across themembrane) along the membrane was taken into account; therefore, thepermeance of the component i, P_(m,i), was defined as:$P_{m,i} = \frac{J_{i}}{\Delta \quad P_{\ln,i}}$ where${\Delta \quad P_{\quad {\ln,i}}} = \frac{\left( {\Delta \quad P_{i}} \right)^{I} - \left( {\Delta \quad P_{i}} \right)^{I\quad I}}{\ln \left\{ {\left( {\Delta \quad P_{i}} \right)^{I}/\left( {\Delta \quad P_{i}} \right)^{I\quad I}} \right\}}$

[0088] where J_(i) is the steady-state flux of component i through themembrane. (ΔP_(i))^(I) and (ΔP_(i))^(II) are the partial-pressuredifferences of component i between the tube-side and shell-sidepressures of the membrane at the gas entrance (I) and exit (II) ends,respectively.

[0089] The separation factor defined by the ratio of permeabilities canbe equivalent to the ratio of permeances if the membrane thickness isidentical. Thus, for pure-gas permeation, the ideal separation factor α₁could be defined by the ratio of permeances of individual pure gases.Analogous to the definition of α_(I), the true separation factor α_(t)of mixed gas is defined by the ratio of permeances of constituent gases.

[0090] 2.B.—Ultramicroporous Silica Membrane with Microporous C6STSSublayer

[0091] A coated membrane support as produced in 1.C. utilizing the C6STSsol as prepared in 1.B.1. is characterized using a single gaspermeability measurement system. The membrane ends were sealed usingViton® or Grafoil® gasket material, and the compression of the gasketavoided by-passing of the gases. A custom built automated flow system,as described above in 2.A. was used to measure the permeance(flux/pressure) of five different gases through the membrane. The gaseswere chosen to give a range of gas molecule sizes. These gases, alongwith their characteristic diameters, are: He (2.65 Å), H₂ (2.89Å), CO₂(3.3 Å), N₂ (3.64 Å), and CH₄ (3.8 Å). Apart from the different sizes,the inert gases have different chemical interactions with the membranesurface. Thus, the flow through the membrane will be a combination ofKnudsen diffusion, surface diffusion and micropore diffusion. Therelative contribution from each of the above flow mechanisms variesaccording to the gas, as well as the pore size of the membrane.

[0092] The flow through the membranes was measured with two bubblemeters installed on the exhaust line. The results obtained from thesingle gas permeability measurements were reported as permeance(cm³/cm²-s-cm-Hg) vs measurement temperature and the ideal selectivityα_(½)(flux of pure gas 1/flux of pure gas 2) vs. temperature.

[0093] The C6STS sublayer as prepared serves to eliminate intrinsicdefects on commercial porous supports and promotes pore uniformity;therefore increasing selectivity, and preventing penetration of asubsequently deposited ultramicroporous membrane (e.g. A2** membranewith an average pore size of about 3-4 Å as described in 1.D.), thusenhancing flux. Gas permeances and selectivities were compared for A2**membranes with and without a C6STS sublayer. Referring to FIG. 5A and5B, at 60° C., the membrane with a sublayer exhibited four-fold higherCO₂ permeances and four-fold higher CO₂/CH₄ selectivities than thatwithout a sublayer in a single-component gas permeation measurement.Ideal separation factors of various gas pairs (e.g. α_(I)(CO₂/CH₄)=102at 25° C.) largely exceeded Knudsen separation factors (e.g.α_(K)(CO₂/CH₄)=0.6). The exceptional negative activation energy of CO₂(E_(a)=-3.37 KJ/mol) for the membrane with a C6STS sublayer indicatedthe occurrence of CO₂ capillary condensation. Therefore, CO₂ istransported with high density through narrow pores at lowertemperatures. On the contrary, CO₂ capillary condensation wasinsignificant for the membrane without a C6STS sublayer. Therefore, agradual increase in CO₂ permeance with temperature (activated transport)is observed (E_(a)=2.48 KJ/mol). Thus, a high CO₂ permeance (3.2×10⁻⁴cm³(STP)/s/cm²/cmHg) and a high CO₂/CH₄ separation factor ofapproximately 200 can be achieved with the dual layer membrane at 26° C.for separation of 50/50 (v/v) CO₂/CH₄ gas mixture, as shown in FIG. 6.The combination of high permeance and high selectivity exceeded that ofprior art gas separation membranes (e.g. asymmetric polyimide with atypical CO₂/CH₄ separation factor of 55 and a CO₂ permeance of 1.7×10⁻⁴cm³(STP)/s/cm²/cmHg). Moreover, activation energies of CH₄ for both A2**membranes with and without a sublayer were 12.92 and 9.64 KJ/mol,respectively, which were higher than those of CO₂. CH₄ diffused throughboth the membranes with low permeance via an activated transportmechanism. Referring back to FIG. 5A, the combination of CO₂condensation effect and the activated transport mechanism help inexplaining the increase in ideal separation factor of CO₂/CH₄ upon thedecrease in temperature for the membrane with a sublayer.

[0094] 2.C.—BTE/TOES Microporous Silica Membrane with a MesoporousC16STS Sublayer

[0095] A membrane support prepared as described in 1.C. utilizing themesoporous C16STS material as the surfactant-templated silica sublayeras prepared in 1.B.2. shows substantially the same substrate effects asthe microporous C6STS materials described above in 2.B. A 5.5 cmcommercial 50 Å γ-Al₂O₃ membrane tube was initially cleaned byultrasound, then washed and calcined as described in 1.C. The membranetube was dip-coated into the surfactant-templated sol as described in1.C.1. utilizing the C16STS sol as prepared in 1.B.2. The C16-surfactantwas then removed by heating the C16STS coated support at a rate of 1°C./min. to the target temperature of 500° C. The coated support was heldisothermally for three hours and then cooled at a rate of 1° C./min. to25° C. A microporous membrane was then formed on the calcined membranesupport with the C16STS intermediate layer by dip-coating the C16STScoated support into a BTE (Bis(Triethoxysilyl)Ethane) silica sol asprepared in 1.A.4. and filtered utilizing 0.45 μm filter (Nalgene) priorto dip-coating. The Ethane ligands (—CH₂CH₂—) of the BTE membraneembedded in the silica framework are removed by calcination at 280° C.by heating the BTE coated support at 1° C./min. to the targettemperature. The coated support was held isothermally for three hoursand then cooled at a rate of 1° C./min. to 25° C., leaving behind amicroporous membrane. A comparison of the membranes with and without amesoporous sublayer is shown in FIGS. 7A and 7B. The results show thatmembranes with a mesoporous sublayer exhibit significantly higherpermeance and significantly better selectivity than those without asublayer. This is consistent with the observation in Example 2.B.,demonstrating the crucial role of the sublayer in improving flux andselectivity of an overlying microporous silica membrane.

[0096] 2.D.—Alternative Intermediate Layers

[0097] The same concept of providing a microporous or mesoporousintermediate layer can be applied to the use of an ordered mesoporoussilica membrane as a sublayer to minimize transport resistance. Anamphiphilic block copolymer (Brij56, CH₃(CH₂)₁₅(CH₂ CH₂O)₁₀OH, 4 wt %)was used as a templating agent in replace of surfactants as prepared anddescribed in 1.B.4. The copolyer sol is deposited onto substrate asdescribed in 1.C.1. To facilitate re-organization of the block-copolymertemplate, the coating of the substrate with the copolymer sol isperformed in a humid chamber of 40% relative humidity at a temperatureof 25° C. The block copolymer template silica results in a high porosity(50-65%) and ordered cubic structure having uniform pore size of about23 Å. Therefore, the block-copolymer is another candidate which can beused as the intermediate layer for a subsequently deposited effectiveseparation layer, such as A2**.

EXAMPLE 3

[0098] Effects Of The Ultramicroporous Silica Membrane Calcined At 450°C.

[0099] 3A.—Ultramicroporous Silica Membrane for Hydrogen Purification

[0100] A dual layered membrane as prepared in 1.C.3. was calcined at the450° C. for one hour in air and cooled at a rate of 1° C./min. to 25° C.Single-component gas permeation measurements were taken utilizing themeasurement system as described in 2.A. at various temperatures rangingfrom 20° C. to 150° C. Pure-gas permeation results at 80° C. are shownin FIG. 8. Due to the extended calcination for one hour at 450° C., thepore size of the membrane was further reduced, resulting in a sharpmolecular-size cut-off at about 3.5 Å. The dual layer membrane supportobtained through the extended calcination at 450° C. results in asuperior hydrogen separation factor (H₂/CH₄=1265) as well as having ahigh hydrogen permeance (1×10⁻³ cm³(STP)/cm²/s/cmnHg). Such a selectivemembrane provides a great opportunity in applications such as hydrogenrecovery from petrochemical plants and hydrogen purification for fuelcells. The membrane selectively separated hydrogen from a simulatedreformate gas mixture consisting of 33.98% N₂, 15.00% CO₂, 0.997% CO,50.023% H₂ (composition produced by the partial oxidation of methanol)for fuel cells as evidenced by the high concentration of hydrogenrecovered in the permeate side stream as shown in Table 1. TABLE 1Permeances of components of a gas mixture* at 80° C. Permeance × 10⁶Separation Pure gas (cm³ (STP)/s/cm²/ Factor permeance PermeateRetentate Gas i c/cmHg) (H₂/Gas i) ratio (mole %) (mole %) H₂ 507 — —92.19 42.25 CO₂ 101 5.0 5.2 7.36 16.89 N₂ 2.15 235.9 316.0 0.37 40.47 CO3.83 132.2 198.6 0.0193 1.14

[0101] A 92 mole % H₂ purity was obtained in the permeate stream at astage-cut of 8.2% (stage-cut=the ratio of permeate flow rate to feedflow rate). The CO concentration, (CO is a known PEM fuel cell poison)in the permeate reduced to at least fifty times lower than that in thefeed. A high H₂ permeance (6×10⁻⁴(STP)/cm²/s/cmHg) and a high H₂/N₂separation factor of over 270 for separation of a 50/50 (v/v) H₂/N₂ gasmixture were achieved as shown in Table 2. TABLE 2 The use of a AC450silica membrane for separation of a 50/50 (v/v) H₂/N₂ gas mixture* at80° C. Permeance × 10⁶ Separation Pure gas (cm³ (STP)/s/cm²/ Factorpermeance Permeate Retentate Gas i c/cmHg) (H₂/Gas i) ratio (mole %)(mole %) H₂ 606 — — 99.41 37.43 N₂ 2.21 274.5 316.0 0.59 62.57

[0102] Besides H₂ purification, the membrane can also be applied toNO_(x) removal from fuel gas. A single-component permeation measurementwas also performed in which NO/NO₂ selectivity was measured to be 9.3cm³(STP)/cm²/s/cmHg and NO permeance reached 3×10⁻⁵ cm³(STP)/cm²/s/cmHg.

[0103] 3.B.—Comparison of the Ultramicroporous Silica Membrane

[0104] A dual-layer supported membrane as prepared in 3.A. was furthercompared to prior art molecular sieving methods and devices. Operatingat 26° C., a moderate pressure gradient (ΔP=5.5 bar), and high feed flowrate (˜500 cm³ (STP)/min), a high CO₂ permeance of about 3.2×10⁻⁴cm³(STP)/s /cm²/cmHg and a high CO₂/CH₄ separation factor of over 200was achieved for the separation of 50/50 (v/v) CO₂/CH₄ gas mixture, asshown in FIG. 9. Still referring to FIG. 9, the A2** membrane asprepared in 1.C.2. without further calcination, results in a combinationof high permeance and high selectivity which is superior to prior artseparation membranes. The A2** membrane as prepared in 3.A. with furthercalcination results in a higher CO₂/CH₄ separation factor (˜600) at theexpense of CO₂ permeance. The comparisons are made under the assumptionthat the best membrane performance was reported under their optimaloperation conditions for each membrane.

[0105] EXAMPLE 4

[0106] Permeance And CO₂CH₄ Separation Factor As Function Of Feed FlowRate, Temperature And Pressure Gradient

[0107] The effect of temperature, flow rate and pressure gradient acrossthe membrane on both CO₂ permeance and CO₂/CH₄ separation factor wasinvestigated via mixed-gas permeation experiments. Mixed-gas permeationexperiments representing the true separation were conducted with 50/50(v/v) CO₂/CH₄ feed gas under steady-state conditions.

[0108] 4.A.—Feed Flow-Rate Effects

[0109] A 5.5 cm commercial 50 Å γ-Al₂O₃ membrane tube prepared asdescribed in 1.C. was dip-coated into the surfactant-template sol asdescribed in 1.C.1. utilizing the C6STS sol as prepared in 1.B.1. TheC6STS coated membrane was then dip-coated in an A2** sol as described in1.C.2. The experiment for feed flow-rate effect was operated at aconstant temperature of 26° C. and a constant pressure gradient ofΔP=5.5 bar across the membrane. The stage cut θ, defined as the ratio ofpermeate flow rate to feed flow rate, represents the fraction of feedgas permeated through the membrane (gas recovery ratio). Referring toFIGS. 10A and 10B, it is shown that the stage cut θ decreases with theincrease in the feed flow rate. The decrease in the stage cut θ resultedin the increase in the CO₂/CH₄ separation factor, α(CO₂/CH₄), as shownin FIG. 10A. Thus, higher purity of CO₂ was recovered at the permeateside at the lower stage cut θ. In a high feed flow rate (low stage cut),the concentration of the fast permeating gas, CO₂, remained high atretentate, as shown in FIG. 10B. Therefore, a high CO₂ driving forceacross the membrane was maintained. In addition, CO₂ is allowed to passthrough pores with little chance of being blocked by the larger CH₄molecules. Alternatively, the CH₄ driving force across the membraneincreases with the decrease in feed flow rate, resulting in a lowerα(CO₂/CH₄). High α(CO₂/CH₄) can be obtained at high feed flow rates butat the expense of both low gas recovery ratio and pumping energy. Themaximum α(CO₂/CH₄) at a fixed temperature and pressure in the tubularmembrane can be achieved when the retentate compositions are equivalentto the feed compositions. Moreover, due to selective adsorption of CO₂at lower temperatures, the mixed-gas α(CO₂/CH₄) is higher than thesingle-component separation factor, α₁(CO₂/CH₄), under identicaloperating conditions. The selectively adsorbed CO₂ would either coverthe pore entrance or further reduce the size of the pore opening,depending on the size of pores, thus preventing CH₄ from permeatingthrough the pores.

[0110] 4.B.—Temperature Effects

[0111] A dual-layer membrane as prepared in 4.A. was subjected to aconstant feed flow-rate represented by a stage-cut of 9% and a constantpressure gradient of ΔP=5.5bar across the membrane while the temperaturewas varied from 26° C. to 120° C. The value of the CO₂ permeance and theseparation factor, α(CO₂/CH₄), are inversely related as the temperatureincreases. FIG. 6 shows that the α(CO₂/CH₄) decreases with the increasein temperature, while the CO₂ permeance increases slightly with theincrease in temperature. The abrupt increase in the α(CO₂/CH₄) at lowertemperatures was due to the combined effects of activated transport andCO₂ selective adsorption where the capability of CO₂ adsorption on tosilica surfaces diminishes at higher temperature. Due to the CO₂selective adsorption effect, the α(CO₂/CH₄) for mixed gas at lowertemperature is higher than the CO₂ separation factor for asingle-component gas, α₁(CO₂/CH₄). The difference between α(CO₂/CH₄) andα₁(CO₂/CH₄) vanishes at higher temperatures as can be seen in comparingFIG. 5A to FIG. 6.

[0112] 4.C.—The Effects Of Pressure

[0113] The effects of pressure on CO₂ permeance through a dual-layermembrane was determined. The membrane, as prepared in 4.A., wassubjected to a constant feed flow-rate, represented by a stage-cut of9%, and a constant temperature of 26° C. The retentate pressure wasvaried while permeate pressure was kept at ambient atmospheric pressure.Referring to FIG. 11 it can be seen that the α(CO₂/CH₄) is essentiallyindependent of the pressure gradient while the CO₂ permeance slightlyincreases with pressure gradient and then levels off at a higherpressure gradient. The increase in CO₂ permeance at the lower pressuregradient is indicative of the startup of CO₂ adsorption inside thepores. Once the CO₂ is adsorbed on the pore walls, the CO₂ permeanceobeys Henry adsorption characteristics such that the CO₂ permeance isindependent of the pressure gradient across the membrane.

[0114] 4.D.—Effects of Continuous Operation

[0115] A membrane, as prepared in 4.A., was continuously operated for150 hours at a constant temperature, pressure gradient, and feed flowrate conditions. Referring to FIG. 12, the CO₂ permeance and separationfactor, α(CO₂/CH₄), gradually increase with time. This is due to agradual removal of adsorbed pore fluid by an extensive gas permeation.Nevertheless, stable and continuous operation is achieved at atemperature of 26° C.

[0116] The foregoing description of a specific embodiment of the presentinvention is presented for the purposes of illustration and description.It is not intended to be exhaustive or to limit the invention to theprecise form disclosed; obviously many modifications and variations arepossible in view of the above teachings. The embodiment was chosen anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents.

What is claimed is:
 1. An inorganic microporous supported membrane,comprising: a macroporous support; a templated porous intermediate layercoating the support; and a microporous layer coating the templatedporous intermediate layer such that the microporous layer is capable ofmolecular sieving.
 2. The inorganic microporous supported membrane asclaimed in claim 1, wherein: the templated porous intermediate layer isan inorganic surfactant-templated silica layer; and the microporouslayer is an inorganic silica layer.
 3. The inorganic microporoussupported membrane as claimed in claim 2, wherein: thesurfactant-templated porous intermediate layer having an average poresize of less than about 20 Å; and the microporous layer having anaverage pore size of less than about 5 Å.
 4. The inorganic microporoussupported membrane as claimed in claim 1, wherein: the templated porousintermediate layer including an amphiphilic block copolymer.
 5. Aninorganic microporous supported membrane, comprising: a poroussubstrate; a first inorganic porous membrane coating the substratehaving an average pore size of less than about 25 Å; and a secondinorganic porous membrane coating the first inorganic membrane having anaverage pore size of less than about 6 Å.
 6. The inorganic microporoussupported membrane as claimed in claim 5, wherein: the first inorganicporous membrane has a pore diameter in a range of about 10 to 20 Å. 7.The inorganic microporous supported membrane as claimed in claim 6,wherein: the first inorganic porous membrane includes asurfactant-templated material.
 8. The inorganic microporous supportedmembrane as claimed in claim 7, wherein: the surfactant-templatedmaterial is prepared from silica polymers and a surfactant powder. 9.The inorganic microporous supported membrane as claimed in claim 5,wherein: the second inorganic porous membrane is an inorganic silicamembrane.
 10. The inorganic microporous supported membrane as claimed inclaim 5, wherein: the first inorganic porous membrane has a thickness ofless than about 100 Å.
 11. The inorganic microporous supported membraneas claimed in claim 5, wherein: the second inorganic porous membrane hasan average pore size of between 2 and 5 Å.
 12. The inorganic microporoussupported membrane as claimed in claim 5, wherein: the second inorganicporous membrane has an average pore size of between 3 and 4 Å.
 13. Theinorganic microporous supported membrane as claimed in claim 5, wherein:the second inorganic porous membrane has a thickness less than about 100nm.
 14. The inorganic microporous supported membrane as claimed in claim5, wherein the porous substrate is an alumina substrate.
 15. Theinorganic microporous supported membrane as claimed in claim 6, wherein:the porous substrate has an average pore diameter ranging between 30 to60 Å.
 16. A method for producing an inorganic dual-layered microporoussupported membrane capable of molecular sieving, the method comprising:contacting a porous substrate with a surfactant-template polymeric solresulting in a surfactant sol coated membrane support; drying thesurfactant sol coated membrane support producing a surfactant-templatedpolymeric coated substrate; calcining the surfactant-templated polymericcoated substrate to produce an intermediate layer surfactant membrane;contacting the intermediate layer surfactant templated membrane with asecond polymeric sol producing a polymeric sol coated substrate; anddrying the polymeric sol coated substrate producing an inorganicpolymeric coated substrate; calcining the inorganic polymeric coatedsubstrate producing the inorganic dual-layered microporous supportedmembrane.
 17. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 16, wherein: the stepof calcining the inorganic polymeric coated substrate includes:calcining the inorganic polymeric coated substrate at a firsttemperature producing a dual-layered supported membrane; furthercalcining the dual-layered supported membrane at a second temperature toproduce the inorganic dual-layered microporous supported membrane. 18.The method for producing the inorganic dual-layered microporoussupported membrane as claimed in claim 17, wherein: the step ofcalcining the inorganic polymeric coated substrate includes calciningunder a vacuum.
 19. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 18, wherein: the stepof calcining the inorganic polymeric coated substrate includes calciningunder a vacuum of less than about 6 pounds per square inch absolute(psia).
 20. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 18, wherein: the stepof calcining the inorganic polymeric coated substrate includes calciningunder a vacuum of less than about 4 psia.
 21. The method for producingthe inorganic dual-layered microporous supported membrane as claimed inclaim 17, wherein: the step of calcining the inorganic polymeric coatedsubstrate includes calcining at a temperature ranging from 200 to 400°C.
 22. The method for producing the inorganic dual-layered microporoussupported membrane as claimed in claim 21, wherein: the step ofcalcining the inorganic polymeric coated substrate includes calcining ata temperature ranging from 250 to 350° C.
 23. The method for producingthe inorganic dual-layered microporous supported membrane as claimed inclaim 17, wherein: the step of calcining the dual-layered inorganicsupported membrane includes calcining at a temperature ranging from 300to 600° C.
 24. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 23, wherein: the stepof calcining the dual-layered inorganic supported membrane includescalcining at a temperature ranging from 400 to 500° C.
 25. The methodfor producing the inorganic dual-layered microporous supported membraneas claimed in claim 24, wherein: the step of calcining the dual-layeredinorganic supported membrane includes calcining for between about 30 to90 minutes.
 26. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 16, wherein: the stepof calcining the inorganic polymeric coated substrate includes calciningunder a vacuum.
 27. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 26, wherein: the stepof calcining the inorganic polymeric coated substrate includes calciningunder a vacuum of less than about 6 pounds per square inch absolute(psia).
 28. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 27, wherein: the stepof calcining the inorganic polymeric coated substrate includes calciningunder a vacuum of less than about 4 pounds per square inch absolute(psia).
 29. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 16, wherein: the stepof heating the surfactant-template membrane substrate includes heatingat a temperature between 500 to 600° C.
 30. The method for producing theinorganic dual-layered microporous supported membrane as claimed inclaim 29, wherein: the step of heating the surfactant-template membranesubstrate includes heating for between about 30 to 90 minutes.
 31. Themethod for producing the inorganic dual-layered microporous supportedmembrane as claimed in claim 16, wherein: the step of calcining thesurfactant-templated polymeric coated substrate includes calcining at atemperature between 100-150° C.
 32. The method for producing theinorganic dual-layered microporous supported membrane as claimed inclaim 16, wherein: the surfactant-template polymeric sol comprisessilica polymers.
 33. The method for producing the inorganic dual-layeredmicroporous supported membrane as claimed in claim 16, wherein: thesecond polymeric sol comprises silica polymers.
 34. The method forproducing the inorganic dual-layered microporous supported membrane asclaimed in claim 16, wherein: the surfactant-template polymeric sol isprepared and deposited under conditions of low condensation rate; andthe second polymeric sol is prepared and deposited under a condition oflow condensation rate.
 35. The method for producing an inorganicdual-layered microporous supported membrane as claimed in claim 16,wherein the method is performed under Class 100 clean room conditions.36. The method for producing an inorganic dual-layered microporoussupported membrane as claimed in claim 16, wherein the step of dryingthe surfactant sol coated membrane support is performed under conditionsof low relative pressure of the liquid constituents.
 37. A method forproducing a supported membrane capable of molecular sieving, comprising:preventing a subsequently deposited top microporous sol from penetratingfurther into the support including: modifying a surface of the support;and depositing the top microporous membrane on a modified supportproducing the supported membrane capable of molecular sieving.
 38. Themethod for producing a supported membrane as claimed in claim 37,wherein: the step of modifying a surface of the support includes:depositing an intermediate membrane on the surface.
 39. The method forproducing a supported membrane as claimed in claim 38, wherein: the stepof depositing the intermediate membrane includes depositing an inorganicsurfactant-templated silica intermediate layer having an average poresize of less than 25 Å.
 40. The method for producing a supportedmembrane as claimed in claim 38, wherein: the step of depositing theintermediate membrane includes: depositing a surfact-template sol ontothe support; drying the surfactant sol coated support; calcining thedried surfactant sol coated support resulting in a surfactant supportedmembrane.
 41. The method for producing a supported membrane as claimedin claim 38, wherein: the step of depositing an intermediate membraneincluding: removing the surfactant-template by heating the surfactantsupported membrane producing the modified support.
 42. The method forproducing a supported membrane as claimed in claim 38, wherein: the stepof depositing the top microporous membrane includes: depositing aninorganic polymeric sol on the modified support; drying the polymericsol coated support resulting in an inorganic polymeric sol coatedsupport; calcining the inorganic polymeric sol coated support resultingin a dual-layered supported membrane; further calcining the dual-layeredsupported membrane resulting in the supported membrane capable ofmolecular sieving.